FINDING AND RECOGNITION OF THE SNAIL INTERMEDIATE HOSTS BY 3 SPECIES OF ECHINOSTOME CERCARIAE

Finding and recognition of snail second intermediate hosts was studied in cercariae of 3 echinostome species. The cercariae of the 3 species accumulated in snail-conditioned water (SCW) with 2 types of orientat ion mechanisms and responded to different small molecular weight (< 50 0 Da) components of SCW. Pseudechinoparyphium echinatum and Echinostom a revolutum cercariae returned by swimming an are, when swimming in de creasing concentration gradients of SCW (turn-back swimming). The stim ulating cues of SCW were identified as hydrophilic organic molecules, probably posessing amino groups. Amino acids contributed to the attrac tivity of SCW, at least in P. echinatum, but they could not account fo r the complete attractivity of SCW. Hypoderaeum conoideum were directe d chemotactically and swam along increasing concentration gradients of small peptides within SCW, but in decreasing SCW gradients they showe d no turn-back swimming. Chemotactic orientation in H. conoideum only started 1 h after emission, which may assist the cercariae to leave th e immediate area of their first intermediate host snails and to disper se. Attachments occurred specifically to snail hosts in the 3 species and were stimulated by macromolecular mucus compounds, probably mainly by viscoelastic properties of the mucus. The results of this study sh ow, that host-finding mechanisms and the stimulating host cues of snai l invading echinostome cercariae differ considerably from those of sch istosome miracidia.